The exploration for and discovery of new oil reserves has become increasingly challenging and costly. Untapped reserves tend to be more difficult to identify and evaluate, and are often located subsea, which further increases the complexity and cost of discovering such reserves. Successful, efficient, and cost effective identification and evaluation of hydrocarbon-bearing reservoirs is therefore very desirable.
In marine exploration, seep detection has become an important tool to identify potential hydrocarbon resources in the subsurface. Oil and gas accumulations often leak hydrocarbons including methane, ethane, propane, butane, naphthalene, and benzene. These hydrocarbons may migrate toward the surface (i.e., the seafloor), through a variety of pathways, such as faults or fracture zones. As such, the seeps become surface expressions of subsurface geological phenomena and can be used to give an indication of the subsurface conditions. In some instances, seeps may not be directly above the accumulation from which they originate but rather have further migrated and mixed with the sea water.
Analysis of fluid and sediment samples that are collected from, in, and around hydrocarbon seeps can be used to determine the presence of a mature source rock. However, such analysis cannot confirm or disprove whether the hydrocarbon seep is also connected to a hydrocarbon reservoir. That is, in some instances, while the seep is emanating from a source rock, the source rock may not be connected to a hydrocarbon reservoir. As such, there is a desire to for methods that enable one to determine whether or not a hydrocarbon seep is connected to a hydrocarbon reservoir.
The microbial ecology of a hydrocarbon seep can provide additional information that may be used to characterize the hydrocarbon reservoir from which the seep emanated. That is, it may be possible to use biological information from the hydrocarbon seep for exploration and hydrocarbon characterization purposes. For example, PCT Publication No. WO 2013/119350 describes using the community function and community structure of a sample ecology from a hydrocarbon seep to determine the location of a hydrocarbon reservoir. Additionally, U.S. Patent Application Publication No. 2006/0154306 describes using genotypic analysis of a sample for the presence of thermophilic or extremophilic microorganisms and comparing the biological profile of the sample to those from reference samples to determine the type of oil, quality of oil, gas/oil ratio, depth, or migration route of the sample. Further, U.S. Pat. No. 8,071,295 describes methods for performing surveys of the genetic diversity of a population, creating a database comprising the survey information, and analyzing the information to correlate the presence of nucleic acid markers with desired parameters in a sample, where the surveys are useful in the fields of geochemical exploration, agriculture, bioremediation, environmental analysis, clinical microbiology, forensic science, and medicine.
However, much of the work used to obtain biological information from hydrocarbon systems has relied on culture-based techniques. These techniques are limited because many of the organisms, particularly those living within a hydrocarbon reservoir, are not able to be cultured. While identifying and finding microbes that have originated in the reservoir and have been transported to the surface would be ideal, it is likely that only a limited number of the microbes would possibly survive transport intact. Thus, relying on culture-based techniques may not be feasible or provide a full representation of the subsurface biodiversity.
In addition, past studies have assumed that organisms living in the subsurface are similar to those at the surface. However, recent evidence indicates that the biodiversity in the subsurface is quite complex and many the subsurface species found have not been identified previously. Thus, with increasing genetic divergence from known reference species, the use of “lab-on-a-chip” type tools (e.g., microarrays using oligonucleotide-type probes or polymerase chain reaction (PCR) methods) that require specific binding of probes to identify certain known target biological species becomes less effective. That is, many of the probe-based methods may be restricted to finding organisms that have some genetic similarity to known organisms, and therefore can potentially fail to identify a large portion of the species within a sample.
Application of microbiology-based tracers has also been used to identify whether thermogenic hydrocarbons are present, by examining where hydrocarbon degradation occurs or is associated with known functions such as bacterial sulfate reduction or reactions that alter fluid properties. However, while these methods are useful for identifying diagnostic organisms or probes associated with a particular function, they do not provide information about the in situ conditions, such as the pressure, temperature, or salinity, within the reservoir.
Therefore, it would be desirable to have improved methods of using biological information from the hydrocarbon seep for exploration and hydrocarbon characterization purposes. For example, there is a need for improved methods for determining whether a hydrocarbon seep is connected to a hydrocarbon reservoir and for methods to characterize the hydrocarbon reservoir from which the seep emanated. For example, there is a need for improved methods for determining in situ conditions of a hydrocarbon reservoir, such as determining whether the reservoir is connected to a hypersaline aquifer, and thus whether the reservoir is a hypersaline reservoir.
Additional background references may include U.S. Patent Application Publication Nos. 2010/279290, 2011/0118983, 2012/0158306, 2013/0157275, 2014/0227723, 2014/0315765, and 2015/0291992; PCT Application Publication Nos. WO 2010/109173, WO 2012/016215, WO 2015/103165, and WO 2015/103332; GB Patent Application Publication No. 2478511 A; Chinese Patent Application Publication Nos. CN 102154453, CN 104630336, and CN 104651350; Lazar et al., “Distribution of anaerobic methane-oxidizing and sulfate-reducing communities in the G11 Nyegga pockmark, Norwegian Sea, Vol. 100, No. 4, pp. 639-653 (July 2011); Orphan et al., “Culture-Dependent and Culture-Independent Characterization of Microbial Assemblages Associated with High-Temperature Petroleum Reservoirs”, Applied and Environmental Microbiology, Vol. 66, No. 2, pp 700-711 (February 2011); and Waldron et al., “Salinity Constraints on Subsurface Archaeal Diversity and Methanogenesis in Sedimentary Rock Rich in Organic Matter”, Applied and Environmental Microbiology, Vol. 73, No. 13, pp 4171-4179 (July 2007).